Fossil Focus: Seals, sea lions and walruses

Introduction

Pinnipeds (seals, sea lions and walruses) make up the second most numerous group of marine mammals (behind whales), with 35 species found throughout the world’s oceans and in several freshwater lakes. They are represented by 3 living families: the Phocidae (earless seals; 19 species), found around the globe; the Otariidae (sea lions and fur seals; 15 species), restricted to the North Pacific and Southern Hemisphere; and the Odobenidae (walrus; 1 species), confined to the Arctic. Two additional extinct families are recognized: the Desmatophocidae and the Enaliarctidae of the North Pacific. Pinnipeds show a variety of adaptations for aquatic living, including flippers for moving through water, changes in teeth related to capturing and feeding on slippery fish and squid and adaptations to allow deep diving. They differ from the other major groups of marine mammals (for example, manatees and whales) in that although they are highly adapted to an aquatic existence, they must return to land for breeding. This has led to the development of elaborate breeding behaviours and vast differences in body size between males and females in many species (a phenomenon referred to as sexual dimorphism). Fossil pinnipeds have been identified from every continent apart from Antarctica (Fig. 1), with a fossil record extending at least as far back as the late Oligocene epoch (around 27 million years ago, Fig. 2).

Figure 1 — Locations of fossil pinniped discoveries around the world. Map produced using the online palaeogeographic map creator tool designed by John Alroy and available at fossilworks.org. Data used to generate map available at www.paleobiodb.org.Figure 2 — Consensus phylogeny of Pinnipedimorpha. Solid vertical lines represent recorded stratigraphic ranges of lineages through the Cenozoic period (from 66 million years ago to today), from first to last appearance. Dashed lines represent hypothetical stratigraphic ranges. Question marks represent uncertain relationships; names in parentheses represent groups that may not share common ancestry.

Seal origins

The origin of the pinnipeds and their relationships to other carnivores have been controversial. All studies agree that pinnipeds belong to the Arctoidea, a group of carnivores that contains bears, weasels and raccoons. Traditionally, pinnipeds were considered to have separate origins within Arctoidea, with odobenids and otariids more closely related to bears, whereas phocids were thought to be more closely related to otters. More recently, independent studies of genes and bones indicate that the different pinniped families are each other’s closest relatives, and that they share a single origin.

The closest living relatives of pinnipeds are still debated. Studies that rely on genes have found that the Musteloidea, a group of carnivores that contains weasels, skunks, raccoons and red pandas, are the closest living relatives of pinnipeds. By contrast, studies that have looked at anatomy suggest that pinnipeds’ closest living relatives are bears. Even less is known about which fossil carnivore groups are most closely related to pinnipeds. Possible close fossil relatives include the otter bear Kolponomos, the otter-like semantorids and Puijila, and the bear-dog Amphicyonodon.

Fossil seal diversity

The oldest creatures known for sure to be pinnipeds belong to the Enaliarctidae. This group does not represent a true family, but rather an assemblage of fossil groups, or taxa, ancestral to the later pinniped families (Figs 2 and 3). This group includes the fossil seals Enaliarctos, Pteronarctos, Pacificotaria and Pinnarctidion; all of these species are known from the latest Oligocene and early Miocene epoch (30-16 million years ago, Fig. 2) of Washington, Oregon, California, and Japan. Enaliarctids possess a variety of features found in later-occurring pinnipeds, such as flippers, but still have teeth similar to those of terrestrial carnivores.

Figure 3 — Block containing the skeleton of Enaliarctos mealsi, one of the oldest known enaliarctine seals. Specimen housed at the National Museum of Natural History in Washington D.C. Photograph by author.

The other extinct group of pinnipeds is the Desmatophocidae, which inhabited the North Pacific from the early to late Miocene (9-23 million years ago, Fig. 4). This family includes the relatively well-known fossil seals Allodesmus and Desmatophoca. They were the first pinnipeds to achieve massive body sizes and may have filled a niche similar to that of modern sea lions and elephant seals. They are believed to be closely related to earless seals, although other studies have argued for a closer relationship with the group that includes sea lions and walruses.

Figure 4 — Skull of Allodesmus kernensis, a gigantic desmatophocid seal from the middle Miocene of California. Specimen housed at the Los Angeles County Museum of Natural History in Los Angeles, CA. Photograph by author.

The walruses (Odobenidae) first appear in the early Miocene. Walruses were important parts of North Pacific marine-mammal communities, and unlike today were present in temperate and subtropical waters. Genetic data suggests that they are most closely related to the Otariidae, although studies of anatomy have supported a close relationship with the Phocidae. Through much of the Miocene and Pliocene epochs (around 23 million years ago to 2.6 million years ago; Fig. 2) they were the dominant pinniped family in the North Pacific. They are represented by three main groups. The oldest is the imagotarines (Fig. 5), an assemblage of walruses that were superficially similar to sea lions, and gave rise to the other two main groups of walruses. This group includes a wide variety of fossil taxa, ranging from the relatively small and sea-lion-like Proneotherium, Neotherium and Imagotaria, to the gigantic Pontolis. These walruses were restricted to the Miocene of the North Pacific. A second, now extinct, radiation of walruses, which evolved during the late Miocene, was made up of the dusignathines, represented by Gomphotaria (Fig. 6) and Dusignathus. These were very large, and their most distinctive features were prominent tusks in both the upper and lower tooth rows. The dusignathine walruses survived through the Pliocene, restricted to the North Pacific. The final group of walruses, the Odobeninae or true walruses, appears in the late Miocene. These walruses possessed prominent tusks used in combat between males. They also show the greatest adaptation towards feeding by sucking the prey into the mouth, with extreme simplification of the teeth, which reaches its extreme in the nearly toothless walrus Valenictus. Unlike the earlier groups of walruses, odobenines dispersed out of the North Pacific, into the Arctic Ocean and the North Atlantic, where they are represented by the living Odobenus and the extinct Ontocetus, respectively.

Figure 5 — An assortment of Miocene imagotariine walruses. From left to right: Proneotherium repenningi, Neotherium mirum and Imagotaria downsi. Specimens housed at the Los Angeles County Museum of Natural History in Los Angeles, CA. Photograph by author.Figure 6 — The giant dusignathine walrus Gomphotaria pugnax, from the late Miocene of California. Specimen housed at the Los Angeles County Museum of Natural History in Los Angeles, CA. Photograph by author.

Fur seals and sea lions first appear in the middle Miocene of the North Pacific and are represented by Eotaria. Otariids diversified in the Pliocene, although they remained comparatively rare as well as small in body size, in contrast to contemporary dusignathine and odobenine walruses. The first otariids may have hunted in the open ocean, based on their rarity and the offshore environments from which fossils have been recovered. Other fossil taxa include the extinct Pithanotaria and Thalassoleon, as well as taxa allied to the extant northern fur seal Callorhinus and Steller’s sea lion Eumetopias. During the latest Miocene and early Pliocene, otariids expanded into the Southern Hemisphere, thanks to increased productivity and falling sea surface temperatures in equatorial waters, where they rapidly diversified.

The oldest earless seals are known from the middle Miocene of the North Atlantic and North Africa, represented by Leptophoca, Monotherium and Afrophoca. However, more-controversial fossils exist for the late Oligocene. Unlike the above pinniped groups, phocids are completely unknown from the North Pacific before the latest Pliocene. Two main groups of phocid are known, and both were already present during the middle Miocene. These groups are the Monachinae, which are represented by the elephant, monk, and Antarctic seals; and the Phocinae, which include bearded, hooded, and harbour seals. The North American and European phocid fossil record is relatively sparse and poorly studied, and many species are known only from isolated limb bones. Their fossil record is better in the Southern Hemisphere, which phocids reached during the Miocene. Excellent fossil material exists for the South American Acrophoca (Fig. 7), Piscophoca, and Hadrokirus, as well as the African Homiphoca.

Figure 7 — The South American fossil seal Acrophoca longirostris, one of the best-known fossil earless seals. Specimen housed at the National Museum of Natural History in Washington D.C. Photograph by author.

Pinniped breeding behaviour

One of the most fascinating aspects of modern seal biology is the variety of breeding behaviours that different groups use. Some seals, such as elephant seals and northern fur seals, use elaborate harem breeding systems, where males battle for control of territory and groups of females (Fig. 8). In seals that use these breeding systems, males are much larger than females. Such differences in body size, termed sexual dimorphism, have been a frequent topic of interest in pinniped palaeoecology. Other seals do not have harems, and the males and females are of similar size. Some seals use breeding displays to attract mates; examples include the elaborate nose sacks of hooded seals (Cystophora cristata), or the underwater vocalizations of Weddell seals (Leptonychotes weddellii).

Figure 8 — Male versus male combat over mates is a common breeding behaviour in seals, such as these New Zealand sea lions (Phocarctos hookeri). Evidence from the fossil record suggests that fossil seals had similar behaviour. Photograph by author.

Sexual dimorphism has been documented in some fossil walruses, earless seals, desmatophocids, and, most recently, the earliest known pinniped relatives, Enaliarctos. This suggests that harem breeding and male versus male combat for mates appeared very early in pinniped evolution. It also suggests that harem breeding has evolved independently multiple times across different pinniped groups.

Diet and foraging behaviour of fossil seals

Much research has focused on interpreting the diet and feeding behaviour of fossil pinnipeds. Unlike their land-living relatives, pinnipeds have simplified teeth, which are of no use in chewing. Instead, teeth are used in feeding only to hold prey. This is referred to as pierce feeding.

In general, four main methods of feeding have been recognized. The most common type is raptorial feeding, in which pinnipeds use their teeth to bite and hold prey before swallowing it whole. This is present in a wide variety of fossil seals, including desmatophocids and enaliarctids, and probably represents the earliest feeding style for the group.

The second most common feeding method is suction feeding, in which prey is sucked directly into the mouth, with no use of teeth. Several pinnipeds show extreme adaptations towards this feeding style, especially the living walrus, although it is likely that most seals can also suction feed small prey to some extent. Suction feeding has been identified in many fossil walruses, and suggested for the fossil desmatophocid Allodesmus.

Filter feeding is found in the living crabeater seal (Lobodon carcinophaga), which uses specialized teeth to strain krill from the water column. Finally, grip-and-tear feeding is restricted to the living leopard seal (Hydrurga leptonyx), which uses its sharp teeth and prominent canines to rip chunks of flesh from larger prey, including penguins and other seals. Neither of these two feeding styles has been documented in fossil pinnipeds.

Pinniped palaeontology: unresolved questions

Although every year brings new fossil discoveries and studies that clarify the evolution and biology of fossil seals, much is still unknown. For instance, we do not have a good understanding of which terrestrial carnivores in the fossil record are most closely related to seals. This has prevented scientists from addressing where seals originated, as well as whether they evolved in tropical or cold climates, or freshwater or marine environments. The evolution of some seal groups is still poorly known, in particular for fur seals and sea lions as well as earless seals. Finally, many basic questions regarding the origins of aquatic adaptation still need to be addressed, ranging from the origin of the unique ‘caterpillar-like’ movement of earless seals on land to the role of environmental change in influencing seal diversity and extinction, and the evolution of deep diving and sensory adaptations for underwater foraging. Future work will address these concerns, and provide a more complete picture of the evolution of modern marine mammal communities.